The present invention relates to combustion furnaces and particularly to improved seals for sealing the entry to such furnaces.
Analyzers for elements, such as carbon, sulfur, and nitrogen, require the combustion of specimens, typically solids or liquids, in a holder, such as a crucible or combustion boat. In many instruments, the holder for the combustion sample is a ceramic holder. Horizontal furnaces, such as described in U.S. Pat. No. 5,064,617 issued Nov. 12, 1991, to O'Brien et al., the disclosure of which is incorporated herein by reference, utilize a ceramic combustion boat which is extended within an electrically heated furnace for combusting samples within a ceramic combustion chamber to temperatures of from about 1000° C. to about 1500° C. U.S. Pat. No. 5,314,662, issued May 24, 1994, to Heinz et al., discloses one system for automatically sequentially loading specimens into such a furnace and includes a mechanism for the admission and withdrawal of a combustion boat for an analysis. The disclosure of the U.S. Pat. No. 5,314,662 patent is also incorporated herein by reference.
With the increasing interest in the accurate determination of low concentration levels of elements, such as nitrogen, by various industries, the sealing of the furnace's entry way against atmospheric contaminants is becoming increasingly important. Although O-ring face-type seals have been employed in the past as has the use of purging gases in a purge block holding the combustion sample prior to entry into the furnace, such seals still can allow the admission of some atmospheric nitrogen, particularly where a push-rod is employed which extends through the purge block to position a sample holder, such as a combustion boat, into the combustion furnace itself. Also, the combustion furnace must be opened for the removal of a spent combustion boat and the admission of a new specimen in a new combustion boat. This process also allows the admission of atmospheric nitrogen and other atmospheric contaminants which must then be purged from the system.
Although existing furnace seals and purge blocks provide acceptable results, with the increasing industry demand for the measurement of low concentrations of analytes such as nitrogen, they are often limited by how the atmospheric nitrogen content impacts the blank measurement. As an example, if the blank nitrogen level was 50 ppm as when UHP oxygen (99.995%) is used as a carrier gas during an analysis, nitrogen levels in the surrounding atmosphere is more than 15,000 times the nitrogen level found in the blank. Thus, in order to achieve low level blanks, all atmospheric contamination must be avoided or eliminated. Thus, not only must the atmosphere be purged for the sample compartment but seals used to isolate the sample compartment from the surrounding environment must be highly effective. This is a major challenge where the seals must be cycled open during each sample to introduce new samples into the purge block sample-holding compartment.
In addition, sliding seals employed with push-pull rods for the introduction and withdrawal of samples from a furnace also present a challenge for maintaining effective isolation of the combustion chamber from the surrounding atmospheric environment. Small imperfections or scratches in the sliding rod can allow environmental contamination into the chamber and any other leaks likewise contaminate the specimen to be combusted and analyzed.